Process to manufacture an ion-permeable and electrically conducting flat material, the material obtained according to the process, and fuel cells

ABSTRACT

A fibrous, flat and ion-permeable material made of synthetic fibers, in particular of synthetically spun fibers such as acrylic fibers or aramid fibers, is processed into staple fibers of a specific length and then fibrillated. In a wet-laid inclined machine (paper machine), the fibrillated fibers are formed into a continuous web and then the web or portions of it are subjected to a temperature treatment to make the web electrically conducting by carbonizing/graphitizing the web through heating.

This invention pertains to a process to manufacture a fibrous, flat andion-permeable material made of synthetic fibers, as well as a materialproduced according to the process, and a fuel cell.

PRIOR ART

A fuel cell is used to convert energy electrochemically into electricalenergy. One known fuel cell is the so-called polymer electrolyte fuelcell, the distinguishing characteristics of which is that a protonicallyconducting, electrically non-conducting polymer membrane is used as thesolid electrolyte (FIG. 1). The solid electrolyte performs the dualfunction of an electrolyte (ionic conductivity via protons, transportnumber 1) and that of a separator (separation of the reactant gaseshydrogen and oxygen). A known polymer electrolyte fuel cell contains acathode and an anode, each of which contains a gas diffusion layer madefrom a carbon fiber web. The cathode and the anode are separated fromone another by the polymer electrolyte membrane, which is electronicallynon-conducting, but nevertheless facilitates ion exchange. Standardmembranes used today largely include perfluorinated membranes, forexample Nafion® made by DuPont Nemours or Femion®, which is made byAsahi Glass. In order to make the perfluorinated membranes electricallyconducting, platinum or platinum alloys are used First of all, this isto prevent corrosion of the electrocatalyst, and secondly to facilitatethe necessary conversion per unit surface (current density) of theelectrochemical reaction at low overvoltages at these relatively lowtemperatures.

In order to keep the material costs of the fuel cell low, platinum issupported on carbon (Pt/C high dispersion) and then applied as a thinlayer, blended with ionomer material, to the membrane or to the gasdiffusion layer through spraying, pouring etc. The electrochemicalreaction takes place at the boundary between the ion-conducting material(membrane) and the electron-conducting catalyst particles. In theprocess, it is important for the catalyst particles to be electricallyconnected to one another. (percolation).

Due to the low perpendicular conductivity of the catalyst layer, it mustbe connected electronically (electron conductance) over its entireactive surface in the requires a mat perpendicular direction to acurrent-supplying gas diffusion layer that acts as a current collector.This erial that on the one hand is sufficiently dense at the associatedconductivity so as form as many points of contact in the catalyst layeras possible, and on the other hand is kept as open as possible (porous)so that the mass flow of the reactants can proceed from the rear of thegas diffusion layer. The gases are distributed through the channels ofthe bipolar plate from the rear and flow through the cell parallel tothe rear of the gas diffusion layer, with a portion of the gas beingtransported perpendicular to this plane (flow direction) through the gasdiffusion layer to the active layer.

On the cathode side, the reaction product, “water”, must be dischargedin fluid form. This requires that the gas diffusion layer have certain“non-wetting” surface characteristics so that the gas diffusion layerdoes not flood. This is accomplished by impregnation with e.g. a PTFEsuspension followed by tempering. On a mass basis, PTFE loads of up to30% are typical. This results in a distribution of so-called hydrophobicpores (for gas) and hydrophilic pores (for water), which is importantfor the functioning of the gas diffusion layer. It is not required thatthe gas diffusion layer on the cathode side be identical with that onthe anode side.

The manufacture of the permeable membrane (=carbon cloth) is veryexpensive. For example, known support materials used are carbon fibercloths, baths, webs or similar.

EP 0 834 936 discloses a non-woven fabric made of inorganic and organicfibers for separators of non-aqueous electrolyte batteries, which isproduced by a wet paper making process. The non-woven fabric has athickness non-uniformity index (Rpy) of 1000 mV or less in machinedirection. Further, the fabric has a center surface average roughnessSRa of 6 □m or less in whole wavelength region measured by athree-dimensional surface roughness meter The fabric contains organicpolypropylene, polyethylene, polymethypentene or acrylic fibers, whichhave the function of heat fusion bonding fibers. In addition, the fabriccontains heat resistant fibers selected from the group of aramid fibers,polyphenylene sulfide fibers, polyarylate fibers, polyether ketonefibers, poyimide fibers, polyether sulfone fibers andpoly-p-phenylenebenzobisoxazole fibers having a melting point or heatdecomposition point of 250° C. or higher. For manufacturing the fabricthe organic fibers are cut to lengths of 1 to 30 mm, preferably to 5 mmor less, and the raw material is dispersed in water in a concentrationof up to 25%, preferably in a concentration between 1 and 10%. Thesuspension thus formed is repeatedly passed through a high pressurehomogenizer for splitting the fibers parallel to the fiber axis(fibrillation). The fabric is manufactured by a wet paper making processand is then subjected to a hot calendaring treatment at a temperature of50-200° C. At this temperature range no carbonizing of the fibres takesplace. The non-woven fabric has a weight of 5-100 g/m², preferably 10-50g/m².

U.S. Pat. No. 3,047,455 relates to the manufacture of paper or nonwovenproducts comprising randomly intermingled discontinuous fibers which areat least in part composed of highly fibrillated synthetic non-cellulosicfibers of paper-making lengths U.S. Pat. No. 3,047,455 teaches the useof wet spun synthetic, in particular acrylic fibers. Wet spun synthetichave a coarse, sponge-like structure and can surprisingly be used formanufacturing paper. The wet spun fibers are cut to staple lengths,suspended in water and battered in a conventional beater, whereby thefibers are fibrillated. The beaten fibrillated acrylic fibers arethereafter formed into a paper product by any suitable process usingstandard paper mill equipment. The paper products are then dried at atemperature ranging between room temperature and the temperature atwhich the acrylic polymer degrades or melts.

The objective of this invention is to propose a process by which aporous, flat, electrically conducting and ion-permeable material can beproduced cost effectively, and preferably in a continuous process.Another goal is to prepare a flat, ion-permeable material that can beused in fuel cells in particular. A further object is to provide a fuelcell with improved gas diffusion layers, which can be manufactured morecost-effectively and in a continuous process.

DESCRIPTION OF THE INVENTION

As specified by the invention, in a process according to the preamble ofclaim 1, staple fibers of a specific length are first fibrillated, thenformed into a continuous web by means of a paper machine, preferably inan inclined wet-laid wire machine, and the web or sections thereof aresubjected to a calendaring process und subsequently to a temperaturetreatment to obtain its electrical conductivity bycarbonizing/graphitizing. The process according to the invention permitsa gas-permeable material to be manufactured cost-effectively that can beemployed as a gas diffusion layer in polymer electrolyte fuel cells.Surprisingly, it has been successfully shown that it is possible tomanufacture a micro porous material made of synthetic fibers using thewet-laid paper-making manufacturing process of forming a fibrous web orfelt, and to make this fibrous material electrically conducting, i.e.ion-permeable, by subsequently converting the synthetics tocarbon/graphite. This is in contrast to the prior art, according towhich carbon fibers are employed who are already electrically conductiveand to process these into a flat material or layer.

According to the conventional process, carbon fibers are processed intoan open non-woven web having a pore size of typically >100 μm. In orderto obtain the desired micro porosity with pores <5 μm, the flat,wide-pore material is impregnated with carbon powder. The disadvantageof an impregnation with carbon powder, however, is that the surface ofthe diffusion layer is not smooth, but rather has a texture thatcorresponds to the particle size of the powder. In contrast thereto andaccording to the invention the desired microporosity can be obtained byusing a paper-making manufacturing process.

The material manufactured according to the invention can perform thesame function as the known gas diffusion layers used in polymerelectrolyte fuel cells. The process according to the invention has thetechnical and economical advantage of being able to form a micro porouscontinuous web material cost-effectively in a continuous productionprocess employing relatively simple technical means at efficientproduction speeds.

Advantageously, the carbonization or graphitizing process takes place ata temperature of greater than 600° C., preferably greater than 800° C.,and very much preferred greater than 1000° C. In these temperatureranges the polymeric organic fibres can readily be transformed intocarbonized/graphitized fibres, which are electrically conductive.

Advantageously, the web is melted at least partially by a firsttemperature treatment that at least partially softens or melts thefibres and forms said web and precedes the carbonizing/graphitizingtemperature treatment (second temperature treatment). The advantage inthis is that the web develops a more dense and less porous cover layeron its surface. By appropriately selecting the temperature and pressureduring the calendaring process and the degree of fibre fibrillation atthe employed fibres, the desired micro porosity of the layer, inparticular of a cover layer being integral with the web, can beachieved. It is to be understood that the web can be made from two ormore single webs and laminated to a single web.

It is advantageous for the staple fibers selected to have a cut lengthof between 4 and 40 mm, preferably between 8 and 12 mm and beingpreferably of a size of 0.5 dtex to 3 dtex.

It is preferred to fix the flat material in a tenter frame prior to thecarbonization/graphitizing process. To the surprise of the inventor, thepore size does not change notably during the carbonization process. Itis advantageous to suspend the staple fibers in a solvent, preferablywater, to form a pulp or pulp of fibres to be fibrillated. Thefibrillation (formation of small frays on the fibers) is best performedin a refiner, preferably a Jones Refiner. It is advantageous if theportion of staple fibers by weight in the pulp, i.e. the pulp dilutionin the refiner, that is fibrillated in the refiner is betweenapproximately 0.1 and 0.01 weight percent, preferably between 0.05 and0.02 weight percent. Good results were obtained with these fractions.

A mixture of fibrillated and non-fibrillated fibers can be used to formthe webs. This permits the porosity of the web to be controlled. Thewebs can have a specific or substance weight of typically between 45 to150 g/m². It is advantageous to use fibers with a Titer of up to 15 dtexmaximum, preferably up to a maximum of 8 dtex, and especially preferredwith a Titer up to a maximum of 3.0 dtex. Preferably, the Titer of thefibres used ranges between of 0.5 dtex and 3 dtex.

According to an advantageous embodiment variant, synthetic fibers of atleast a first and a second type are used. These fibers can consist ofchemically different synthetic materials or can contain additives. Thesefibres may be different e.g. as to their composition, stability and/ormelting point. Thus, a portion of the synthetic fibers used can containa noble metal, for example platinum or gold. The noble metal can havethe function of a catalyst.

It is advantageous to calendar the flat material or web at least onceprior to carbonization. This can result in a densification of the upperlayer, especially if the calendaring process is carried out at increasedtemperatures, and preferably simultaneous with the first temperaturetreatment. It is preferred to calender the material at least twicebefore carbonization, and such that the first calendaring step densifiesall of the material and the second calendaring step modifies one or bothof the paper surfaces into a film-like, micro porous material bysoftening the fibrillated fibers and creating a film-like micro-poroussurface or cover layer of the web. In the process, the effect of theheat and the pressure can be selected such that the calendared materialhas the desired pore size afterward, for example <5 μm, preferably <2μm. Non-crystalline synthetic fibers, for example acrylic, polyacrylateor aramid fibers, can be employed.

The object of this invention is also the provision of a fibrous, flat(two-dimensional) and porous material obtained via a process accordingto one of claims 1 through 19.

A further object of this invention is a non-woven fabric, whichcomprises or essentially consists of carbonized/graphitized polymericfibres, in particular such a fabric having pore sizes of less than 10 μmand preferably less than 5 μm, and most preferably less than 2 μm. Saidfabric can be coated with a catalyst layer and/or with a electricallynon-conductive, ion-permeable membrane (PEM). In a preferred embodimentthe the fabric has a core having a first porosity and at least one coverlayer having a second porosity, said second porosity being less porousthan the first porosity.

A further object of this invention is a fuel cell with at least two gasdiffusion layers that are separated by means of an electricallynon-conducting, but proton permeable separating wall or membrane, andthat can be layered with at least one catalyst such as platinum, saidfuel cell being characterized in that the gas diffusion layers are madeat least in part of a material according to one of claims 20 to 22 andnon-woven fabric according to one of claims 23 to 29. The object of thisinvention is also the use of a material obtained according to one ofclaims 1 through 19 as a microporous support for a membrane, inparticular for a proton exchange membrane (PEM).

The invention is described in more detail below with reference to theattached figures. Shown are:

FIG. 1 A sketch showing the principals of a fuel cell with aproton-permeable membrane (polymer electrolyte membrane=PEM)

FIG. 2 A sketch of a known gas diffusion layer made of carbon fibers;

FIG. 3 The design of a known polymer electrolyte fuel cell with two gasdiffusion layers separated by an ion-permeable membrane, shownschematically;

FIG. 4 The principal of a polymer electrolyte fuel cell with two gasdiffusion layers according to the invention, shown schematically; and

FIG. 5 A cross section through the novel, flat material producedaccording to the process and described invention.

A known fuel cell 11 has two electrodes, an anode 17 and a cathode 19,which are placed at or attached to the opposite surfaces of aproton-permeable, electrically non-conducting membrane 21. Hydrogen isoxidized at the anode 17 and the hydrogen ion that arises from theoxidation passes through the proton-permeable membrane (PEM) 21 andreaches the cathode 19. The electrons make their way through theexternal, closed electrical circuit 23 to the cathode 19, performingelectrical work in the process. At the cathode 19, each hydrogen ionabsorbs an electron and reacts to form water in the presence of oxygen.

FIGS. 2 and 3 show a known polymer electrolyte fuel cell in more detail.The cathode 19 and anode 17, each of which has a gas diffusion layer 25a, are made of a micro porous material that is permeable to the reactantgases hydrogen and oxygen as well as to water. The conventional gasdiffusion layers 25 a are produced from a carbon fiber web 27 that isimpregnated on one side with a carbon impregnation 29. The carbonimpregnation 29 consists essentially of carbon/graphite dust. The carbondust performs the additional function of providing the desired microporosity of the gas diffusion layers 25 a. On top of the carbonimpregnation 29 is a platinum layer 30 that acts as a catalyst. Insteadof a separate platinum layer 30, platinum can also be mixed in with thecarbon dust of the carbon impregnation 29 as a catalyst in order to makethe layer electrically conducting.

In contrast with the known polymer electrolyte fuel cells, the gasdiffusion layer 25 b of the cell according to the invention is producedfrom fibrillated synthetic fibers 31. The surface of the layer is moredense on at least one side (cover layer 33) than in the rest of thelayer (FIG. 4). The surface is densified preferably through calendaringat a specific increased temperature. In this way, a micro porousmaterial can be obtained that is permeable to hydrogen and oxygen. Oneadvantage to the gas diffusion layer according to the invention is thatthe surfaces are very smooth so that platinum can be applied as afilm-like, but porous layer.

FIG. 5 shows a cross section through a novel, flat material produced bythe process according to the invention. The material has a centralfibrous and porous core 35 and micro porous cover layers 33 on thesurfaces that are more dense than the fibrous region 35. The fibrouscore 35 and the fibrous cover layers 33 are made up essentially ofstaple fibers of a specific length. The staple fibers of the coverlayers 35 are more dense, preferably as a result of single or multiplecalendaring, and are partially softened.

The process to manufacture the flat, electrically conducting material isdescribed in more detail below by means of example:

To produce staple fibers, synthetic fibers, preferably synthetic acrylicfibers, are first cut to a specific length, preferably between 8 and 12mm. Then, a pulp is made consisting of the staple fibers and water. Itis advantageous to fibrillate the fibers in a Jones refiner at amaterial density of approximately 0.5 to 0.02%. It has been shown thatfibers with a Titer of 1.2 dtex to 3.0 dtex and cut-lengths from 8 to 12mm are best suited to provide a good fibrillation result and a goodsheet structure. The dimensions of the fibrils depend on the polymerstructure. In the case of acrylic fibers, it has been observed thatfibrils of up to 2 mm in length occur and have a diameter ofapproximately 0.2 μm. The more fibrils that can be produced at theindividual fibers, the denser the fiber cloth becomes. It isadvantageous for the refiner to have a cutting angle of <5° and thecutting surface gaps should not exceed ⅔ of the fiber length. Thematerial of the refiner cone can be made of metal or also basalt.

A portion of the above fibers is left in their original, non-fibrillatedstate, being later mixed into the pulp of the fibrillated fibers. Thisincreases the porosity of the middle layer of the finished material. Asecondary effect is the increase in stiffness and tensilestrength/working strength. The non-fibrillated fibers can also beso-called sheet core fibers. Here, the non-fibrillated fiber can have aneven smaller fiber diameter than the fibrillated fiber, so as to preventthe formation of larger pores. The fibrillated fibers are furtherdiluted after treatment in the refiner, and mixed with other types offibers if necessary, for example those that support a catalytic process.The dilution helps to prevent the formation of fiber bundles, flocks andknots. The dilution also helps the fibers to deposit evenly when laterforming the paper web in the subsequent inclined-wire-head box. Moredilution of the stock takes place downstream of the machine [stock]chest on the way to the head box such that the final needed dilutionlevel of the fibrous material of 0.0004% to 0.00015% in water isachieved prior to it. This extreme dilution is advantageous in ensuringan even, highly uniform fiber distribution on the paper machine wire bymaking sure each fiber is deposited individually. Commonly, dewateringand sheet formation is done at the inclined wire inside the head box. Atthe outlet lip of the inclined-wire-head-box, the web then appears inits final consolidated form. The subsequent dewatering of the paper webthrough suction under the machine wire after the head box, as performedon a fourdrinier type paper machine (PM), is not needed when employingan inclined wire-head box, commonly used for forming wet-laid-nonwovens.

The now finished paper web leaves the paper machine wire and movesfreely along the supporting transport wire of the drying section. Thiscan be a flat bed flow-through dryer, in which the air stream fixes theweb onto the filter and the remaining water between the fibers is driedby the air passing through it. Because the fibers are fibrillated, andbecause the fibers had matted during the sheet formation in thehead-box, the web has sufficient internal strength that it can be pulledfreely by itself from support roll to support roll and densified in acalendar. It can then be rolled up.

What is novel is that a paper product of this type, after suitablerepeated calendaring, is in such a form that a micro-porous filtering orseparating material arises, from which an electrically conductingseparating material and gas diffusion layer can be achieved bysubsequently carbonizing/graphitizing the entire material. Thecarbonized material described here can then be used as a micro-poroussupport for a PEM membrane (proton exchange membrane) or the like. Themembrane material can then be refined with a catalyst layer or the likeso as to acquire additional functions.

The fibrillation of the fibers and the subsequent treatment in thecalendar is important for the formation of the pores. In particular, thetemperature control of the calendar, and the overall work energy balanceof the calendar, must be taken into account. The energy balance must beestablished according to the polymeric structures, and is an importantparameter in the reproducibility of the product. For example, it hasbeen shown for an acrylic fiber paper with a weight of 60 g/m² that aninitial calendaring at approximately 85° C. and a line load of 60-70kp/cm and a second calendaring at 105° C. to 120° C. and a line load of75 kp/cm results in a reproducible pore size of <2 μm. The speed of thepaper web was 12 to 24 m/min during these tests. In the secondcalendaring pass, a film-like skin formed on the surface of the papersince the acrylic becomes plastic as a result of the added energy.However, due to the initial fibrillation of the fibers, this skinremains micro-porous. Beneath it, then, is a layer of non-melted fibers,which however were densified very compactly in the calendaring. Thisporous middle layer fills fully with the substance to be separated anddistributes it very evenly to the second melted layer on the oppositeside, or the bottom of the paper, from which it can then exit.

Experiments have demonstrated that the pores do not change, or onlyslightly, if the paper had been fixed in a tenter frame prior to thecarbonization process. The carbonization of the web, i.e. of the flatmaterial, can proceed in stages at temperatures between 600° C. and1400° C. It is preferred that the final full carbonization take place attemperatures above 1000° C. and in particular above approximately 1150°C., preferably at approximately 1250° C. The necessary electricalconductivity for use in fuel cells can be produced throughcarbonization.

Any calendar can be used for the calendaring provided that it can applythe necessary work energy to deform the fibers (work energy=papertemperature+heat added+line pressure+drive power). The roll coatingmaterials can consist of cotton or other fibrous materials (for examplepolyimide, aramid, mixed with other fibers and also as coated fibers,for example with a sputtered aluminum layer) It is recommended that asmany of the parameters as possible be maintained as control parametersof the calendar being used in order to guarantee reproducibility.However, it has also been shown that results can be obtained with a pairof rolls steel on steel. The experiments were done using a multi-rollcalendar and the results showed that it is of no consequence to thetechnical result whether a calendar has only two nips or has more ofthem. Multiple nips have the advantage of higher productivity and betterquality assurance, and these have been known processes for a long timealready in classical high densified capacitor—paper manufacturing.

The finished calendared paper made from synthetic fibers has a milkyappearance and exhibits some opacity. Calendaring can approximatelytriple tensile strength in comparison to the state of the material afterit's dried. The raw density is 0.65 to approximately 0.99 g/cm³. Thepaper is rolled up onto a core. To convert the paper into a carbonproduct, the paper is cut into sheets. These sheets can be held inframes made of ceramic materials in order to place the paper fixed intoan autoclave. The heat treatment process in an autoclave can be doneanalogous to a heating process for the production of carbon fibers.These cloths, or sheets, which are now in the form of a microporouscarbon product, can now be provided with a catalyst coating and besubjected to other coatings or refinements.

A number of known polymer materials can be fibrillated, and not justthose mentioned, but crystalline polymers, such as PET, cannot befibrillated. The general process of forming a pulp has been known for along time and is described in the technical literature, as has the factthat paper can be produced from it with the help of traditionalpaper-making machines.

A fibrous, flat and ion-permeable material made of synthetic fibers, inparticular of synthetically spun fibers, such as acrylic fibers oraramid fibers, is processed into staple fibers of a specific length andthen fibrillated. In a wet-laid inclined wire machine (paper machine),the fibrillated fibers are formed into a continuous web, and the web orsections thereof are subjected to a temperature treatment and apreferably simultaneous calendaring process. The temperature treatmentmelts the staple fibers at least partially so that more densemicro-porous layers result on the surface. The webs, which consist atleast in part of electrically non-conducting synthetic fibers, are madeto be electrically conducting by carbonizing (graphitizing) the web,i.e. the electrically non-conducting synthetic fibers, under heat.

A fibrous, flat and ion-permeable material made of synthetic fibers, inparticular of synthetically spun fibers such as acrylic fibers or aramidfibers, is processed into staple fibers of a specific length and thenfibrillated. In a wet-laid inclined machine (paper machine), thefibrillated fibers are formed into a continuous web and then the web orportions of it are subjected to a temperature treatment to make the webelectrically conducting by carbonizing/graphitizing the web throughheating.

Legand:

-   11 Fuel cell-   17 Anode-   19 Cathode-   21 Proton-permeable, electrically non-conducting membrane-   23 Electrical circuit-   25 a Conventional gas diffusion layer-   25 b Gas diffusion layer according to the invention-   27 Carbon fiber web-   29 Carbon impregnation-   30 Platinum layer-   31 Synthetic fibers of the gas diffusion layer 23 b-   33 Denser, micro-porous cover layer of the gas diffusion layer 25 b-   35 Fibrous, porous core of the gas diffusion layer 25 b

1. A process to manufacture a fibrous, flat and electronicallyconducting material made of synthetic fibers, in particularsynthetically spun fibers (e.g. acrylic fibers), comprising the steps offirst fibrillating staple fibers having preferably a specific length;forming the fibrillated staple fibers into a continuous web in a papermanufacturing process, preferably by means of an inclined wire wet laidpaper machine, characterized in that, the continuous web is calendaredat least once prior to its carbonization and then carbonized/graphitizedthrough heating at a temperature of greater than 600° C., to obtainelectrical conductivity.
 2. A process according to claim 1,characterized in that the carbonization takes place at a temperaturegreater than 800° C., and very much preferred greater than 1000° C.
 3. Aprocess according to claim 1, characterized by an initial firsttemperature treatment that at least partially softens or melts thefibres.
 4. A process according to claim 1, characterized in that theflat material is fixed in a tenter frame prior to the carbonizationprocess.
 5. A process according to claim 1, characterized in that thestaple fibers are suspended in a solvent, preferably water, to form apulp and are then fibrillated.
 6. A process according to claim 1,characterized in that the fibers are fibrillated in a refiner.
 7. Aprocess according to claim 5, characterized in that the pulp dilution inthe refiner is approximately 0.1 to 0.01%, preferably 0.05 to 0.02%. 8.A process according to claim 1, characterized in that a mixture offibrillated and non-fibrillated fibers is used.
 9. A process accordingto claim 1, characterized in that the fibrillated fibers are processedinto webs with, a substance weight typically between 45 to 150 g/m². 10.A process according to claim 1, characterized in that fibers with aTiter of up to 15 dtex maximum, preferably up to 8 dtex maximum andespecially preferred with a Titer of up to 3.0 dtex maximum are used.11. A process according to claim 1, characterized in that fibers withcut lengths between 4 and 40 mm, preferably between 8 and 12 mm are usedto produce the continuous web.
 12. A process according to claim 1,characterized in that synthetic fibers of at least a first and a secondtype are used.
 13. A process according to claim 12, characterized inthat the fibers of a second type contain fractions of at least one noblemetal or other additive, e.g. a synthetic additive.
 14. A processaccording to claim 1, characterized in that the calendaring is carriedout at raised temperatures.
 15. A process according to claim 1,characterized in that the web or material is calendared at least twiceprior to the carbonization and such that all of the material isdensified in a first calendaring step and at least one of the twoopposite paper surfaces is changed into a film-like, porous material bymelting the fibrillated fibers in a second calendaring step.
 16. Aprocess according to claim 1, characterized in that the heat andpressure are selected. such that the calendared micro porous materialhas pore sizes of <5 μm, preferably <2 μm.
 17. A process according toclaim 1, characterized in that synthetic fibers such as acrylic orAramid fibers are used.
 18. A process according to claim 1,characterized in that non-crystalline fibers are used as syntheticfibers.
 19. A fibrous, flat and porous material obtained from a processaccording to claim 1 further characterized in that the material has acore having a first porosity and at least one cover layer having asecond porosity, said second porosity being less porous than the firstporosity.
 20. A material according to claim 19, characterized by afibrous core and at least one micro porous flat cover layer on one sideof the material that is more dense than the fibrous region.
 21. Amaterial according to claim 19, characterized in that the surfaces ofthe material opposite one another are micro porous flat cover layersthat are more dense than the fibrous region.
 22. Non-woven fabriccomprising carbonized/graphitized polymeric fibres characterized in thatthe fabric has a core having a first porosity and at least one coverlayer having a second porosity, said second porosity being less porousthan the first porosity.
 23. Non-woven fabric according to claim 22,characterized in that the fabric consists essentially ofcarbonized/graphitized polymeric fibres.
 24. Non-woven fabric accordingto claim 22, characterized in that, the fabric is coated with a catalystlayer
 25. Non-woven fabric according to one of characterized in that,the fabric is micro porous.
 26. Non-woven fabric according to claim 22,characterized in that, the fabric is made from one single web or layer.27. Non-woven fabric according to claim 22, characterized in that, sucha fabric is made from two or more single webs and laminated to a singleweb.
 28. Fuel cells containing at least two gas diffusion layersseparated by an ionically-electrically conducting layer separating wall(PEM membrane), said gas diffusion layers being coated with at least onecatalyst. characterized in that, each gas diffusion layer is formed atleast in part from a material having a fibrous core and at least onemicro porous flat cover layer on one side of the material that is moredense than the fibrous region and a non-woven fabric consistsessentially of carbonized/graphitized polymeric fibres.
 29. Use of amaterial obtained according to claim 1 and a non-woven fabric comprisingcarbonized/graphitized Polymeric fibres characterized in that the fabrichas a core having a first porosity and at least one cover layer having asecond Porosity, said second porosity being less porous than the firstporosity as a micro porous support for a membrane, in particular a PEMmembrane.